1. Field of the Invention
The present invention relates to a method of growing semiconductor layers by mean of vapor-phase epitaxy and, more particularly, to a vapor-phase heteroepitaxial growth method of growing different kinds of semiconductor layers one on top of the other while controlling the thickness of each semiconductor layer monoatomic scale.
2. Description of the Related Art
New functional devices employing quantum well structures have actively been developed recently. In order to make such devices, it is required to control the thickness of each of crystal layers with accuracy of the order of a monoatomic layer. The metal-organic chemical vapor deposition (MOCVD) method is known as a method of making such crystal layers. With the MOCVD method, however, it is difficult to control the thickness of a crystal layer on a monoatomic scale because it controls the thickness of a layer by means of its growth time alone.
For this reason, for the growth of, for example, III-V compound semiconductor layers, a monoatomic layer growth (which means a growth of a crystal layer with thickness controlled on a monoatomic scale) method has been proposed which introduces a group III material gas and a group V material gas alternately into vessel in which a substrate is placed, thereby growing alternating monoatomic like layers on the substrate. This method is now being studied energetically. The point of such a monoatomic layer growth method is that the growth self-stopping mechanism of the group III elements must be manifested in the thickness of a monoatomic layer. To this end, growth conditions, the substrate temperature in particular, must be set in such a way to suppress decomposition in vapor phase of the group III material and accelerate its decomposition and reaction at the substrate surface. The crystallinity of the resultant crystals and the quantities of introduced impurities vary significantly with the substrate temperature. Thus, a semiconductor crystal layer to be grown or a material gas used has a substrate temperature range suitable for the monoatomic layer growth. It is required to grow a semiconductor layer at a substrate temperature in the suitable temperature range.
In general, the temperature range which realizes the monoatomic layer growth and permits good film quality to be obtained varies in relation to semiconductor crystal layers or material gases. Thus, in order to grow different kinds of semiconductor layers subsequently by the use of the monoatomic layer growth technique, it is required to vary the substrate temperature for each of the semiconductor layers. However, it takes a long time to vary the substrate temperature. The growth has to be suspended for a long time at the interface between different kinds of semiconductor layers. The long-time suspension of the growth is liable to introduce impurities, thus degrading the properties of subsequently grown semiconductor layers.
That is, the problem with the conventional monoatomic layer growth method by which different kinds of semiconductor layers are subsequently grown is that the resultant semiconductor layers are poor in quality.